Congenital heart disease is the most common serious birth defect, affecting .8% of liveborn infants. Normal cardiac development depends on complex interplay between genetic and epigenetic factors. In particular, blood flow and cardiac function are essential for cardiac morphogenesis: however, the mechanism by which these mechanical signals are sensed and interpreted remains unclear. Cilia, which are also essential in the development of cardiac LR asymmetry via their function at the embryonic organizer (node), have recently been found to function as mechanosensors in other tubular, fluid-filled organs such as the kidney. We have identified a set of cilia, called cardiac cilia, in the mouse heart at e8.5 - e12.5, corresponding to the time in development extending from the onset of blood flow through valve formation and septation. The goal of this proposal is to define the mechanism by which cardiac cilia function directly in cardiac morphogenesis independent of their role in the generation of LR asymmetry. Mice with immotile, but structurally normal cilia have abnormal positioning of organs along the LR axis. Although intracardiac defects are observed in 7-50% of affected mice, a significant number survive to adulthood with structurally and functionally normal hearts. In contrast, mice with complete absence of cilia or ciliary sensing have severe cardiac defects with 100% penetrance that result in mid-gestational embryonic lethality independent of LR axis development. These observations suggest that cilia are required in cardiac development independently from their function in LR development. We hypothesize that cardiac cilia function as sensors for extracellular signlas such as flow, cardiac function or secreted ligands to affect morphogenesis. In Spec. Aim 1 of this proposal, we will define what cardiac cilia do: are they mechanosensors, hedgehog receptors or motile structures? To this end, the distribution and composition of cardiac cilia will be examined. The role of constitutive ciliary mutations on cardiac development will be evaluated by analysis of the cardiac phenotype of mouse embryos with mutations resulting in defective ciliary motility, ciliary biogenesis or ciliary mechanosensation. The cardiac defects and distribution of cilia will be evaluated in mouse embryos with a mutation resulting in an absent heart beat. The role of cilia in LR development will be distinguished from their intracradiac function. In Spec. Aim 2, we will identify where cardiac cilia exert their effect by using Cre-lox technology to delete cilia specifically from the epicardium, endocardium, pericardium and anterior heart field. Finally, in Spec. Aim 3 we will seek to define how cardiac cilia direct morphogenesis. Here, the downstream signaling pathway(s) connecting ciliary sensing with cardiac morphogenesis will be investigated by analyzing epithelial-mesenchymal transformation, proliferation and hedgehog signaling in mouse embryo hearts with mutations in ciliary function and biogenesis.